This is on the Before You Travel web page, and has been for a while:

Campsite arrival times
The earliest arrival time at most campsites is now 1pm. Please ensure you arrive at this time or later to avoid congestion, blocking access roads or causing possible disruption to public highways. For the most up to date arrival times, please check the individual campsite webpage.

I agree with hja, we arrived in April and 1pm was already posted as the earliest arrival time on the web page.  The wardens on this site could not be more helpful.   The sign says no waiting on the slip road. To do so wouldn't only block access to the campsite, but also Merrose Farm itself and I'm sure they would not be pleased.

Although we still make the 7 hour minimum trip overnight, we head for a large carpark or layby somewhere to lay up for a few hours before aiming for a 1pm arrival time at the site.

The 1pm arrival time is now on almost every site. It’s not a target - you don’t have to arrive exactly at 1. Everyone has their satnav on which tells them what time they will arrive. Don’t wait until you’re 10mins away from the site, by which time you’ve probably left the main big road, to look for somewhere to wait. Stop about an hour away where there’s more likely to be space, and wait until satnav says you’ll arrive after 1. So what if someone gets “your” first choice of pitch - it might not have been empty anyway! Why be stressed before your holiday even starts?